1. Field of the Invention
The present invention relates to a loudspeaker that directly, with high fidelity, converts into an acoustic output digitally-encoded electrical signals that are representative of such output.
2. Description of the Prior Art
Conventional vibrating plate or diaphragm-type loudspeakers are analog devices. In general, because of the inertia of the vibrating diaphragm and for other reasons, such loudspeakers are known to have low efficiency and high distortion characteristics. It is difficult, also, to produce low frequency sounds with a small size speaker. These and other inherent deficiencies limit the extent to which the fidelity of reproduction of diaphragm-type speakers can be improved and their cost of manufacture, assembly and maintenance reduced.
It is known in the prior art to convert audio signals, such as voice or musical signals, into a pulse code modulation (PCM) signal which is then recorded for later reproduction or transmitted to a distant point for reproduction over a telephone line, for example. This enables audio signals to be recorded or transmitted and then reproduced. Specifically, the analog voice signal is sampled at a constant rate, commonly 44 kHz, and a digital word is produced and transmitted at each sampling, the digital word representing the polarity and magnitude of the analog voice signal at the time of sampling. The digital word is converted back to an analog voice signal which is then applied to a conventional speaker. Significantly, however, for sound reproduction, it is necessary to convert the PCM signal into an audio signal. That is to say, before electro-acoustic conversion, a digital-to-analog converter that can accept PCM signals must be provided to convert the PCM signal into an analog signal that the speaker will accept. The use of such a converter not only increases the cost but introduces signal distortion produced by conversion and amplification. Moreover, the system is still subject to distortion and coloration of sound produced by analog loudspeakers as well as their inefficiency.
It has been proposed in the prior art to provide digitally controlled transducers which decode a digitally-encoded signal received serially by code word to drive a single analog speaker diaphragm. In British Pat. No. 1,382,927, the complete specification for which was published on Feb. 5, 1975, the transducer comprises a cantilevered piezoelectric bender plate having a plurality of driving electrodes of varying areas and each of which is energizable independently of the others by an electrical signal the magnitude of which is determined for each code word by the value of the digital signal in a respective bit position thereof. One end of the bender plate is fixed and the other end is connected to a speaker diaphragm. A similar arrangement is disclosed in U.S. Pat. No. 3,947,708 that was issued on Mar. 30, 1976 to John E. Fulenwider. U.S. Pat. No. 3,153,229 that was issued on Oct. 13, 1964 to C. E. Roberts discloses a direct digital transducer employing an electrorestrictive plate providing a digital readout of an analog function without intermediate conversion to an analog number.
Significantly, in the devices of all three of the above patients, digital-to-analog conversion occurs before electro-acoustic transduction.
Microphones for converting acoustic energy, through a single diaphragm, directly to a digitally-encoded electrical signal are disclosed in the following U.S. Pat. Nos. 3,286,032 issued on Nov. 15, 1966 to Elmer Baum; 3,622,791 issued on Nov. 23, 1971 to Patrice H. Bernard; 3,626,096 issued on Dec. 7, 1971; and 3,958,237 issued on May 18, 1976. The acoustic conversion methods employed in these patents rely upon the use of a single diaphragm, and do not lend themselves to "reverse engineering" for use as loudspeakers.
An ionic electro-acoustic transducer employed as a loudspeaker is disclosed in U.S. Pat. No. 3,476,887 that was issued on Nov. 4, 1969 to A. L. Seligson et al. All detailed discussion of this transducer in the specification of the patent is concerned with the transducer as an analog device. A single reference to "digital" modulation is made in Column 2, line 9 of the specification, but there is no discussion or other disclosure of this, in either the specification or drawings.
Ultrasonic transmitting and receiving devices using dielectric transducers are disclosed in U.S. Pat. No. 4,034,332 that was issued on July 5, 1977 to Pierre M. Alais. These devices transmit ultrasonic wave pulses into an outer liquid or solid medium while focusing the pulses in predetermined and adjustable directions or distances and for detecting such wave pulses.
A direct digital loudspeaker with digital-to-analog conversion occurring after electro-acoustic transduction is disclosed in U.S. Pat. No. 4,194,095 that was issued on Mar. 18, 1980 to Toshitada Doi et al., the disclosure of which patent, by reference, is incorporated herein. This loudspeaker depends for its operation upon the switching, that is, the turning off and on, at an ultrasonic rate, of several digital bit related (air) outlet valves. The air outlets comprise horns that are sized to relate to the significance of the digital bits in the coded signals which control them. The air supply includes a pump and a reservoir.
The loudspeaker of U.S. Pat. No. 4,194,095 involves a large number of mechanical parts such as the air pump and the reservoir, output horns, precision valving and piloting mechanism, and multiple air ducts. The valve driving electronics involves several stages of wave shaping to drive the device from a normal serially coded signal in addition to a serial-to-parallel "buffer." Also, the acoustic output is one-sided, providing positive pressure toward the listener, rather than the preferred push-pull mode of operation. Further, the actual overall fidelity of the sound produced by this speaker system would be reduced by the extraneous noise made by the pumping and duct systems.